Semiconductor devices including a silicon film have been widely used for electronic devices, such as display devices, controllers, etc. In general, monocrystalline silicon has high conductivity. Nevertheless, in view of cost and manufacturing processes, an amorphous silicon film or a polysilicon film is usually used as the semiconductor film. However, the amorphous silicon film, which has insufficiently conductivity, is subjected to annealing to improve the property in many cases.
Typically, it is known that thermal annealing is performed on the amorphous silicon film. In the thermal annealing, the amorphous silicon film is exposed to comparatively high temperatures to form the polysilicon film. In general, the amorphous silicon film is exposed to temperatures in the range from 600° C. to 1100° C. for several ten hours in the thermal annealing. It is noted that thermal annealing at a temperature of 500° C. or lower hardly provides effects, and therefore, it is difficult to perform the thermal annealing using a comparatively low-cost and excellently-processable substrate, such as a glass substrate and a plastic substrate, as a substrate for supporting the amorphous silicon film.
Further, laser annealing has been known as another annealing. The laser annealing is performed in such a manner that laser light is irradiated to the amorphous silicon film to form the polysilicon film. In general, the laser annealing can provide comparatively high energy partially to the amorphous silicon film, so that a substrate that supports the silicon film is not exposed to high temperatures as a whole. Accordingly, a glass substrate and a plastic substrate, which have comparatively low thermal resistance, can be used as the substrate.
The laser annealing uses a continuous wave laser or a pulsed laser. In the case using the continuous wave laser, such as an argon ion laser, a light beam having a spot with a diameter of about 100 μm is irradiated to the silicon film, thereby performing scan by the light beam on the silicon film. When irradiation of the light beam melts the silicon film, the silicon film is gradually solidified according to the energy distribution inside the light beam and movement of the light beam to cause crystallization of the silicon film. However, since the spot diameter of the light beam is small, it may take long time generally for the use of the continuous wave laser to thoroughly eliminate defects in the silicon film, which is comparatively wide.
On the other hand, in the case using the pulsed laser, such as an eximer laser, when laser light having comparatively high energy is irradiated to the silicon film, the silicon film is instantly melted. When the silicon film is solidified thereafter, the silicon film is crystallized. In the pulsed laser, while the spot diameter of the light beam can be increased comparatively large with comparatively high maximum energy of the laser light, the spot diameter of the light beam is still not so large relative to the silicon film. Accordingly, it may take long time to thoroughly eliminate defects of a comparatively large area of the silicon film. Further, when the spot diameter is increased simply in order to shorten the period of time, thermal damage to the substrate may accordingly increase. In this case, the use of a substrate with a thickness of smaller than 1 mm, which has been recently employed in general, may cause thermal damage to deform the substrate.
Besides the thermal annealing and the laser annealing, hydrogen plasma treatment has been known as a scheme for defect elimination (see Patent Literature 1). The hydrogen plasma treatment disclosed in Patent Literature 1 is performed on an amorphous silicon film under low pressures (e.g., 150 mTorr). This may terminate dangling bonds of the amorphous silicon film with hydrogen to eliminate bonding defects.